In known gas springs a piston rod passes through one end of a container tube. Adjacent to this one end a sealing system is provided. This sealing system comprises a first axially outer sealing unit and a second axially inner sealing unit. A liquid chamber is defined between the axially outer and the axially inner sealing unit. Due to liquid migration across the axially inner sealing unit pressure differences may occur across the axially inner sealing unit, particularly when the piston rod is provided with a piston unit, and rapid movements of the piston rod with respect to the container tube are to be expected. Such pressure differences can result in destruction of the axially inner piston unit.
For avoiding such destruction German `Offenlegungsschrift` 35 03 537 shows an axially inner sealing unit which is subject to the action of a mechanical spring. This mechanical spring increases the pressure within the liquid chamber.